1. Field of the Invention
The present invention relates to a semiconductor light emitting diode (LED), and more particularly, to a high-efficiency nitride semiconductor LED with a three-dimensional light-emitting stack structure.
2. Description of the Related Art
Recently, display devices and illuminating devices using semiconductor LEDs such as GaN-based LEDs have been actively developed for low power, high efficiency, high color reproduction, fast response speed, and environmental affinity. Since the semiconductor LED does not reach 801 m/W, which is the efficiency of a fluorescent lamp, it does not still replace the fluorescent lamp used for general illumination. The semiconductor LED is used in a light source for a keypad of a mobile phone, an LCD backlight, a camera flash, a traffic light, and so on. Therefore, in order to develop a semiconductor LED for illumination, the low efficiency of the semiconductor LED must be increased. Especially, many attempts have been made to increase the efficiency of the semiconductor LED through an epitaxial growth and an improved package structure.
An internal quantum efficiency is determined by the number of photons generated at an active layer with respect to carriers injected to the LED. The internal quantum efficiency is expressed as a product of an injection efficiency and a radiative efficiency. The injection efficiency is defined by the ratio of carriers injected into quantum well of the active layer to carriers injected into the LED, and the radiative efficiency is defined by the ratio of photons generated by the carriers injected into the quantum well. The internal quantum efficiency is varied according to a peak wave and an operating current density. Specifically, as the operating current density increases, the internal quantum efficiency decreases. It has been known that this phenomenon is caused by carrier overflowing phenomenon and a reduced recombination efficiency due to the increase of a piezoelectric field at the quantum well.
To solve the low internal quantum efficiency, the operating current density is reduced by increasing the area of the semiconductor LED chip. However, if the size of the semiconductor LED chip is reduced, the price of the LED increases and its yield rapidly decreases. Thus, the reduction in the size of the semiconductor LED chip is not an approach suitable for improving the internal quantum efficiency. Recently, in order to increase the area of the active layer without changing the chip size, a method of forming a three-dimensional light-emitting stack structure using a selective epitaxial growth has been introduced.
FIG. 1 is a sectional view of a conventional semiconductor LED. Referring to FIG. 1, a conventional semiconductor LED 10 includes a GaN layer 13 formed on a sapphire substrate 11 and a three-dimensional light-emitting stack structure 20 formed on the GaN layer 13. The three-dimensional light-emitting stack structure 20 is formed in a pyramid shape. The pyramid-shaped light-emitting stack structure 20 may be formed by selectively growing an n-type GaN crystal using a SiO2 mask 15 to form a pyramid-shaped GaN layer 17, and growing an active layer 18 and a p-type GaN layer 19 on the n-type GaN layer 17. By forming the three-dimensional light-emitting stack structure (specifically, the three-dimensional active layer 19), the area of the active layer 19 can be increased without changing the chip size and the operating current density can be decreased.
However, if the three-dimensional (e.g., pyramid-shaped) light-emitting stack structure is formed using the selective growth, the grown three-dimensional structure has a very small size of less than several μm, and a crystal defect occurs at an apex A and a valley B of the pyramid. Since the thickness (d) of the pyramid is very small, the crystal defect at the apex A and the valley B will influence other parts. Therefore, the light-emission wavelength becomes non-uniform and it is difficult to obtain the intended wavelength. In addition, because the thickness (d) of the pyramid is too small, it is difficult to prevent the bad effect due to the crystal defect generated at the apex A or the valley B of the pyramid by using the manufacturing technique.